/*
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
 */
package com.opensource.dbhelp.dbutils;

import java.io.IOException;
import java.io.InputStream;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

/**
 * <code>QueryLoader</code> is a registry for sets of queries so that multiple copies of the same queries aren't loaded into memory. This implementation loads properties files filled with query name to SQL mappings. This class is thread safe.
 */
public class QueryLoader {

	/**
	 * The Singleton instance of this class.
	 */
	private static final QueryLoader instance = new QueryLoader();

	/**
	 * Return an instance of this class.
	 * 
	 * @return The Singleton instance.
	 */
	public static QueryLoader instance() {
		return instance;
	}

	/**
	 * Maps query set names to Maps of their queries.
	 */
	private final Map<String, Map<String, String>> queries = new HashMap<String, Map<String, String>>();

	/**
	 * QueryLoader constructor.
	 */
	protected QueryLoader() {
		super();
	}

	/**
	 * Loads a Map of query names to SQL values. The Maps are cached so a subsequent request to load queries from the same path will return the cached Map.
	 *
	 * @param path
	 *            The path that the ClassLoader will use to find the file. This is <strong>not</strong> a file system path. If you had a jarred Queries.properties file in the com.yourcorp.app.jdbc package you would pass "/com/yourcorp/app/jdbc/Queries.properties" to this method.
	 * @throws java.io.IOException
	 *             if a file access error occurs
	 * @throws IllegalArgumentException
	 *             if the ClassLoader can't find a file at the given path.
	 * @return Map of query names to SQL values
	 */
	public synchronized Map<String, String> load(String path) throws IOException {

		Map<String, String> queryMap = this.queries.get(path);

		if (queryMap == null) {
			queryMap = this.loadQueries(path);
			this.queries.put(path, queryMap);
		}

		return queryMap;
	}

	/**
	 * Loads a set of named queries into a Map object. This implementation reads a properties file at the given path.
	 * 
	 * @param path
	 *            The path that the ClassLoader will use to find the file.
	 * @throws java.io.IOException
	 *             if a file access error occurs
	 * @throws IllegalArgumentException
	 *             if the ClassLoader can't find a file at the given path.
	 * @since DbUtils 1.1
	 * @return Map of query names to SQL values
	 */
	protected Map<String, String> loadQueries(String path) throws IOException {
		// Findbugs flags getClass().getResource as a bad practice; maybe we should change the API?
		InputStream in = getClass().getResourceAsStream(path);

		if (in == null) {
			throw new IllegalArgumentException(path + " not found.");
		}

		Properties props = new Properties();
		try {
			props.load(in);
		}
		finally {
			in.close();
		}

		// Copy to HashMap for better performance

		@SuppressWarnings({ "rawtypes", "unchecked" })
		// load() always creates <String,String> entries
		HashMap<String, String> hashMap = new HashMap(props);
		return hashMap;
	}

	/**
	 * Removes the queries for the given path from the cache.
	 * 
	 * @param path
	 *            The path that the queries were loaded from.
	 */
	public synchronized void unload(String path) {
		this.queries.remove(path);
	}

}